Circuit board and sensing coil positioning system for proximity sensor

ABSTRACT

A proximity switch assembly (10) for signaling the location of a movable member (36) at either of opposite end limits of movement relative to a stationary frame (44,46) includes a compact elongate housing (58) adapted to be mounted on the frame to extend along the path of movement of a power driven member (32), such as the piston rod of a hydraulic cylinder, which shifts the movable member between its end limits. The proximity switch is fixedly mounted in the housing and carries a switch actuating member (68) mounted for sliding movement over a relatively short distance sufficient to move an actuating target (74) into and out of actuating proximity of an air core proximity sensing coil (120) within the housing. A circuit board holder (96) formed of dielectric plastic material nests within the housing to protect, electrically insulate and precisely position components (120, 100, 102) carried thereby.

TECHNICAL FIELD

The present invention relates to an apparatus for detecting the locationof a movable member at either of two end limits of movement and isparticularly directed to an inductive proximity switch of the type thatutilizes an oscillator drive circuit in combination with an inductiontank circuit to generate a target sensing field.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present invention is related to that disclosed and claimed in U.S.Pat. No. 4,664,364 to Lymburner, entitled Proximity Switch Assembly andU.S. Pat. No. 4,543,527 to Schuchmann et al, entitled Proximity SwitchExhibiting Improved Start-Up Characteristics. This application is alsorelated to a copending application U.S. Ser. No. 133,755 entitledInductive Proximity Switch Exhibiting Magnetic Field Immunity, filedconcurrently herewith in the name of John P. Groves, Joseph Lazzaro andGregory L. Nadolski and assigned to the assignee of this application.

BACKGROUND OF THE INVENTION

Proximity switches are generally known in the art and have been widelyapplied to sense the position of a moving object in manufacturingprocesses. Such known proximity switches utilize an oscillator drivecircuit in combination with an induction tank circuit. The tank circuitincludes an induction coil as a means for sensing the presence of anobject such as metal. The induction coil is constructed such that itgenerates a magnetic field in an area surrounding the coil. The magneticfield induces eddy currents in a conductive object which comes withinthe generated magnetic field. Such objects are known in the art astargets. Once a target comes within the magnetic field of the coil,energy is drawn from the induction coil. A typical induction proximityswitch selects components of the oscillator and tank circuit to insurethat oscillations occur when a target is absent from the magnetic fieldof the induction coil. When a target comes within the magnetic field ofthe induction coil, the oscillation amplitude is attenuated due to theloss of energy caused by eddy currents induced in the target. The amountof the oscillation attenuation is directly related to the distancebetween the target and the induction coil.

A predetermined distance between the induction coil and the target isselected as the point where the output of the proximity switch changesan electrical state to indicate the presence of a target. This distanceis known as the switch trip point. A circuit within the proximity switchmonitors the oscillation amplitude and generates a signal at the outputof the proximity switch indicative of the fact that the target has comewithin the trip point distance.

Most inductive proximity switches employ a ferrite cup core and coilassembly as the transducer or sensing element. This is connected to anoscillator, usually operating between 100 and 600 KHz. The frequency isdetermined by the resonant frequency of the coil and a high qualitytuning capacitor. A cup core is preferred because it allows for the fluxfield to be focused in front of the cup core, allowing a further sensingdistance. The cup core, however, is a liability when used in highmagnetic field environments, such as in or near the throat of a welder.The ferrite material saturates in these fields, causing the oscillatorto damp because of the resulting tank Q degradation. This action makesthe oscillator act as though a target is present. Prior art attempts toovercome this problem have employed innovative oscillator designs andoscillator detector and hold circuitry. This approach has been partiallysuccessfully for the reason that the welding field is sinusoidal innature and there is a time in every half cycle when the field is passingthrough zero. During this period, the oscillator is designed to rapidlybuild up and the detector responds rapidly, holding the detected voltageduring the time the magnetic field builds up and damps the oscillator.While this can be accomplished satisfactorily, this type of oscillatoris inherently more complicated and costly. In addition, there is stillan upper limit as to the strength of magnetic field to which the currentcan be made immune.

There is presently an acute need for proximity switches which can beused in conjunction with pneumatic or hydraulic cylinders in automatedmanufacturing to detect when a host cylinder has reached either limit oftravel. In automated manufacturing applications, particularly gangedwelding of work pieces, numerous power actuated devices are employed,the actuation of which must be carefully choreographed for efficientwork piece flow. Typically, separate proximity switches have been usedfor each actuator to separately detect each limit of travel. Because ofthe large number of proximity switches required, low cost and simplicityof design are paramount.

Another problem with prior art proximity switches lies in the fact thatin adapting them for use with high pressure hydraulic cylinders,expensive and time consuming modification to the cylinders themselvesmust be made.

Still another problem of prior art proximity switches lies in the factthat the commercially available switch designs will not functionproperly when in close proximity to the extremely high electromagneticfields generated by industrial welding operations. Shielding suchproximity switches from external sources of electromagnetic energy hasproven to be only partially effective and expensive.

A final problem with prior art proximity switches lies in the inherentdifficulty in providing a reliable mechanism for positioning andaligning the fixed coil with the moving target. Because any variationsin the path of travel of the target or changes in distances betweentarget end of travel points and the coil can alter the proximity switchset point, cycle to cycle repeatability as well as long term stabilityare critical. Such problems are exacerbated by the presence of largeelectromagnetic fields because the coil and target must be confinedwithin a shielded environment but electrically and magnetically isolatedfrom the shielding itself.

BRIEF DESCRIPTION OF THE INVENTION

The present invention provides a new and improved proximity switch ofthe type that utilizes a tank circuit energized by an oscillator drivecircuit. The new apparatus in accordance with the present invention isdesigned to eliminate the need for two switches located at each end of ahost power actuated device such as a hydraulic cylinder. The newapparatus is also designed to operate in close proximity to weldingequipment and other sources of intense electromagnetic radiation and iseffectively immune from the effects thereof.

The proximity detection switch of the present invention comprises meansoperative to change its conductive state in response to receipt of acontrol signal, an inductive circuit element, means to electricallyenergize the inductive element and thereby establish a flux field of agiven amplitude thereabout, the flux field being contained substantiallywithin a shielded cavity surrounding the inductive element defined by asubstantially closed housing assembly. An insulating retainer is nestedwithin the housing cavity and receives the inductive circuit elementwithin a recess formed therein. The proximity switch also includestarget means also located within the shielded cavity which isdisplaceable between a position remote from the inductive element and asensed position within the flux field adjacent or near the inductiveelement so as to effect a change in the field's amplitude, circuit meansgenerating the control signal as a function of detected changes in theflux field amplitude and, finally, a mechanism interconnecting themoving member of the power actuated device and the target means suchthat the target means will move between its remote and sensed positionsin response to displacement power of the actuated device member. Thisarrangement provides a switch in which target position can be accuratelycontrolled and, thus, the sensing distance minimized. Furthermore, useof an air coil provides almost unlimited magnetic field immunity becausethere is no ferrite core to saturate.

In the preferred embodiment, the search coil is entirely closed in ametallic housing. Thus, the coil is protected from external sidetargets. An alternative construction is envisioned however, employing anon-metallic housing in which the coil is protected from the sidetargets by placing the coil in a copper shield which acts as a remoteshorted turn. This arrangement allows an apparatus which can be locatedextremely close to sources of intense electromagnetic energy.

According to another aspect of the invention, the proximity switchfurther includes means for selectively varying the characteristic fieldamplitude in a manner independent of the position of the target means.This arrangement has the advantage of permitting calibration andadjustment of the desired trip distance.

According to another aspect of the invention, the retainer is integrallyformed from injection molded dielectric plastic material defining a thinuniform barrier between the coil and target for precise positioning ofthe coil with respect to the target and simultaneously electricallyinsulating the coil from the target and housing assembly/shield. Thisarrangement has the advantage of providing simple, rugged andinexpensive construction with precise component positioning.

According to another aspect of the invention, the retainer includes anumber of body portions and retention tabs having ramps and steps formedtherein which, during assembly, are momentarily deflected outwardly byan edge of a circuit board and entrap the circuit board against anassociated body portion. This arrangement has the advantage of holdingthe circuit board in place without fasteners or additional manufacturingsteps and simultaneously insulating the circuit board from the housingassembly/shield. Furthermore, this arrangement permits preassembly andcalibration of virtually all of the switch components outside of thehousing assembly.

According to another aspect of the invention, the housing assembly haslaterally spaced wall portions disposed, upon final assembly of theproximity switch, in close lateral proximity to the retention tabs andbody portions disposed laterally opposite therefrom. This arrangementprevents inadvertent release of the circuit board by unintendeddisplacement of the retention tabs by shock or vibration while ensuringthat the circuit board and its components remain spaced/electricallyinsulated from the case.

According to still another aspect of the present invention, the circuitboard contains light processing circuit elements such as LEDs positionedin spaced aligned relation with translucent/transparent windows formedin the housing assembly. This arrangement has the advantage of allowingan optical input/output link between a portion of the circuit within theshielded cavity with its outside environment without physical contact ofthe light processing circuit element with the housing assembly/shield.

These and other features and advantages of the present invention willbecome apparent upon reading the following specification, which, alongwith the patent drawings, describes and discloses a preferred embodimentof the invention in detail.

A detailed description of the embodiment of the invention makesreference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an assembly in cross section of a proximity switch made inaccordance with the present invention in combination with a poweractuated device;

FIG. 2 is a top plan view of the proximity switch of FIG. 1 on anenlarged scale;

FIG. 3 is a bottom plan view of the proximity switch of FIG. 1 on thescale of FIG. 2;

FIG. 4 is a top plan view of the proximity switch of FIG. 2 withportions thereof broken away to reveal internal details thereof;

FIG. 5 is a cross sectional side view of the proximity switch taken onlines 5--5 of FIG. 2 on a further enlarged scale;

FIG. 6 is a schematic representation of the circuit of the proximityswitch of FIG. 1;

FIG. 7 is a cross sectional view of the proximity switch taken on lines7--7 of FIG. 1;

FIG. 8 is a graphical illustration of the equivalent tank impedance atresonance as the steel target approaches the front surface of the coil;

FIG. 9 is a graphical illustration of the variation of trip distanceversus temperature for a given coil;

FIG. 10 is a graphical illustration of the variation of trip distanceversus temperature for an alternative coil for a far set distance;

FIG. 11 is a graphical illustration of the variation of trip distanceversus temperature for the alternative coil of FIG. 10 for a moderateset distance;

FIG. 12 is an exploded perspective view of the cover member of theproximity switch of FIG. 1 to reveal structural detail of componentscontained therein;

FIG. 13 is a top plan view of the circuit board holder of FIG. 12;

FIG. 14 is a cross sectional side view of the circuit board holder takenon lines 14--14 of FIG. 13;

FIG. 15 is a cross sectional end view of the circuit board holder takenon lines 15--15 of FIG. 14; and

FIG. 16 is a cross sectional end view of the circuit board holder takenon lines 16--16 of FIG. 14.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION

Referring to FIG. 1, a proximity switch 10 is shown in assembly with apower actuated device 12. In the preferred embodiment of the invention,device 12 comprises a hydraulic cylinder including a housing assembly 14made up of a cylindrical wall member 16 closed at each end by caps 18and 20. A piston 22 is slidably disposed within cavity 24 defined bywall member 16. Piston 22 carries an annular seal 26 at the interface ofthe circumference of piston 22 and the inner diameter surface of wallmember 16.

Device 12 is controlled by a hydraulic circuit 28 which is in fluidcommunication with portions of cavity 24 on each side of piston 22 toeffect controlled displacement of piston 22 between a first limit oftravel (illustrated) wherein piston 22 is nearly contacting end cap 20and a second limit of travel wherein piston 22 is displaced rightwardlyfrom its illustrated position and is nearly contacting end cap 18.Hydraulic circuit 28 is controlled by a process controller 30 whichreceives device position feedback information from proximity switch 10.The specific design details of hydraulic circuit 28 and processcontroller 30 are not deemed germane to the present invention and aredeleted here for the sake of brevity.

A piston rod 32 is carried by piston 22 and extends axially outwardlyfrom housing assembly 14 through end cap 20, terminating in a bifurcatedyoke 34 which is interconnected with a structural link member 36 througha pin 38. Pin 38 is held in its illustrated position with respect toyoke 34 by C-clips 40 disposed within outwardly opening circumferentialslots 42 formed in pin 38.

Power actuated device 12 also includes parallel housing extensions orrails 44 and 46 extending from end cap 20 and straddling piston rod 32.Rails 44 and 46 have axially directed inwardly opening elongated slots48 and 50, respectively for receiving the opposed outwardly projectingends of pin 38. As can best be seen in FIG. 7, slots 48 and 50 allowpiston 22, piston rod 32, yoke 34, link member 36 and pin 38 to moveaxially between the first and second limits of travel but preventrotation thereof. This arrangement guides and supports the distal end ofpiston rod 32 in straight line movement throughout its entire stroke.

It is contemplated that link member 36 will engage an apparatus employedin a manufacturing process such as a power actuated clamp described inU.S. Pat. No. 4,664,364 the specification of which is incorporatedherein by reference.

Proximity switch 10 nests within an elongated slot 52 within rail 46.Slots 50 and 52 are in register, receiving a pin 54 extending axiallyupwardly from pin 38 within slot 52. Proximity switch 10 is affixed torail 46 by screws 56 or other suitable fastening means.

Referring to FIGS. 2 through 5, the structural details of proximityswitch 10 are illustrated. Proximity switch 10 includes a generallyrectangular elongate housing assembly 58 including a cover member 60 anda base member 62 held in assembly by suitable fastening means such asscrews 64. Members 60 and 62 are preferably formed of metal to shieldinternal components from external electromagnetic fields typicallypresent in many manufacturing processes. However, it is contemplatedthat members 60 and 62 could be formed from plastic or other suitablenonmetallic material if alternative shielding is provided such as bymetal foil cladding. Base member 62 has two downwardly extending guidemembers 66 integrally formed therewith which, in assembly, extend withinslot 52 for positioning of housing assembly 58 with respect to rail 46.Guide members 66 are laterally spaced and receive an elongated carriermember 68 therebetween. Carrier member 68 has two abutment shoulders 70and 72 integrally formed therein extending downwardly into slot 52 ofrail 46. As will be described in detail hereinbelow, abutment shoulders70 and 72 are integrally formed within carrier member 68 proximate theends thereof and extend far enough into slot 52 to entrap the upwardmostend of pin 54. A target member 74 is integrally formed from the end ofcarrier member 68 associated with abutment shoulder 72 extendingupwardly within housing assembly 58 through a slot 76. Slot 76 isaxially extended to permit limited axial displacement of target 74within housing 58. Carrier 68, including abutment shoulders 70 and 72and target 74 are formed from metal.

Carrier 68 is retained in assembly with base member 62 by rivets 78.Rivets 78 include a pan head portion 80 slidably received within anelongated stepped bore 82 formed in base member 62. The portion ofrivets 78 distal from head 80 are received within registering bores 84in carrier 68 and held in place by welding, swaging or other suitabletechniques. Base member 62 has an axially elongated downwardly openingpocket 86 formed therein overlying the upper surface of carrier 68. Abow spring 88 is disposed within pocket 86, having ends bearing againstthe axial ends thereof and bending downwardly in the central portionthereof to bear against the uppermost surface of carrier 68. Thus,carrier 68 is held in assembly by rivet 80 and is permitted limitedaxial freedom of movement determined by the axial extent of slots 82.Spring 88 biases carrier 68 downwardly to prevent rattling and toenhance smooth predictable displacement of carrier 68 with respect tohousing 58.

Cover member 60 has an industrial quality three pin connector 90 mountedthereon for interfacing proximity switch 10 with process controller 30and a source of line voltage. Within proximity switch 10, one of thelines from connector 90 is grounded at 91 to cover member 60 by welding,soldering or other suitable means. The remaining two lines are routed toand electrically interconnected with an electronic circuit 92, shownschematically in FIG. 6, the components of which are carried by acircuit board 94. Circuit board 94 is insulatively held in itsillustrated position by a holder 96 nested within cover member 60.

Through bores 98 are provided in each corner of base member 62 forreceiving screws 56.

Circuit board holder 96 is formed of electrically insulating dielectricplastic or other suitable material such as 6/6 nylon and is generallyrectangular in shape having its upper and lower ends substantially open.Through a series of tabs, abutments surfaces and snap fittings, holder92 operates to retain circuit board 94 in its illustrated position andspaced from housing 58. Essentially all of the discrete components ofcircuit 92 are carried by circuit board 94 including two light emittingdiodes (LED) 100 and 102 which are positioned in register with opticallyclear windows 104 and 106 formed in cover member 60 to permit anoperator to observe certain operating conditions of proximity switch 10,as will be described in detail hereinbelow. Windows 104 and 106 arethrough bores formed within cover member 60, opening within a pocket orrecess 108 formed in the top surface of cover member 60. An opticallytransparent or translucent label 110 is adhesively retained withinpocket 108. Label 110 serves to carry advertising or instructionalindicia thereon while serving as a protective lens covering windows 104and 106.

Cover member 60 has an area of reduced lateral dimension shown generallyat 114 at the end there 14e of nearest target 74. Circuit board holder96 has a narrowed portion 116 integrally formed therewith extendingleftwardly as viewed in FIG. 5 within area 114. Portion 116 defines acavity 118 nestingly receiving an induction coil 120 therein. Alocalized slot 122 is provided in circuit board holder 96 for routing ofleads 124 and 126 from coil 120 to circuit board 94. The left-handmostsurface of portion 116 provides a protective barrier 128 of knownthickness (1 mm) between coil 120 and target 74.

Referring to FIGS. 5, 7 and 12 through 16, the structural details ofcircuit board holder 96 as well as its interfit with cover member 60,circuit board 94 and coil 120 is illustrated. Holder 96 is composed of aframe including two generally parallel laterally opposed side members96a and 96b integrally formed at the ends thereof with interconnectingend members 96c and 96d. Members 96a through 96d form a generallyrectangular frame structure which is open at the top and bottom thereof.Side members 96a and 96b have centrally located inwardly directed stepportions 96e and 96f, respectively, formed therein. A laterallyextending web portion 96g integrally interconnects the centralmostportion of step portions 96e and 96f, coacting therewith to lendrigidity to the entire structure of holder 96. End members 96c and 96dhave integrally formed laterally extending inwardly directed ribs 96hand 96i, respectively, near the vertical midpoint thereof. Web portion96g is generally rectangular in cross section and extends vertically (asviewed in FIG. 14) from a plane common with the uppermost portion ofmembers 96a through 96d downwardly to a plane common with the lowermostsurface of ribs 96h and 96i. Side members 96a and 96b extend downwardlyto a plane slightly above the plane defined by the lowermost surface ofribs 96h and 96i for reasons which will be described hereinabove. Stepportions 96e and 96f extend downwardly to the plane of the lowermostportion of ribs 96h and 96i. Two downwardly extending longitudinallyspaced retention tabs 96j are integrally formed with side member 96a.Likewise, a pair of similarly longitudinally spaced downwardly directedretention tabs 96k are integrally formed with side member 96b. Eachretention tab 96j and 96k includes an upwardly, inwardly directed rampportion 96l extending from the bottom thereof terminating in a laterallyoutwardly directed step 96m.

Retention tabs 96j and 96k are elongated and side members 96a and 96babbreviated with respect to step portions 96e and 96f to providecompliance or flexibility thereto. Upon assembly, circuit board 94 ispressed upwardly with respect to holder 96. The lateral edges 94a ofcircuit board 94 momentarily engage ramp portions 96l of retention tabs96j and 96k and momentarily deflect them outwardly until the lowermostsurface of circuit board 94 extends above step 96m wherein retentiontabs 96j and 96k spring back to their normal or illustrated position. Atthe same time, the uppermost surface of circuit board 94 is inline-to-line contact with the lowermost surfaces of ribs 96h and 96i,web portion 96g and step portions 96e and 96f. As should be nowapparent, once snapped into position, circuit board 94 is rigidly heldin position, as illustrated in FIG. 5, is electrically insulated fromhousing assembly 58 and is protected from subsequent shock andvibration.

LEDs 100 and 102, as well as other discrete circuit components aremechanically mounted and electrically in circuit upon circuit board 94.LEDs 100 and 102 are positioned in register with windows 104 and 106 andare thus optically accessible from the outside environment. Althoughdescribed as an operator indicating light, it is contemplated that alight receiving component such as a photodiode or phototransistor couldlikewise be placed in alignment with windows 104 and 106 for receivingoptical inputs from the outside environment.

Circuit board holder 96 is dimensioned to closely fit within covermember 60. In assembly, the laterally outermost surfaces of retentiontabs 96j and 96k are essentially in line-to-line contact with theinnermost surfaces of laterally opposed walls 60a of cover member 60 andare thus prevented from inadvertent outward displacement after assemblyis complete. Likewise, the outermost surface of end members 96c and 96dare essentially in line-to-line contact with the inner surfaces oflongitudinally opposed end walls 60b of cover member 60 and preventflexure or distortion of holder 96.

Narrowed portion 116 is integrally formed on the outer surface of endmember 96c and is integrally formed therewith to define cavity 118receiving coil 120. Coil 120 is snugly fit within cavity 118 and is thusproperly aligned with protective barrier 128 and prevented fromdisplacement within cavity 118. If severe vibration or shock conditionswere anticipated, it is contemplated that cavity 118 as well as theremaining cavity defined by housing assembly 58 could be potted orfilled with a dielectric insulating material.

As should now be apparent, holder 96 serves to position circuit board94, the components carried thereby as well as coil 120 which ismaintained in precise alignment with respect to target 74 by virtue ofbarrier 128.

As should now be apparent, a substantial portion of the electricalcircuit including coil 120 can be preassembled prior to installationwithin housing assembly 58. This advantage permits near final assemblycalibration and testing prior to installation of cover member 60.

Referring to FIG. 1, piston rod 32 is shown in its fully extended endlimit of movement wherein pin 54 is engaged with abutment shoulder 72and has positioned carrier 68 in its left-handmost limit of travel. Withcarrier 68 in the position shown in FIG. 1, its target 74 is beyond theactuating proximity of coil 120 of the proximity switch and the switchis in its unactuated or switch off position. When piston rod 32 isdriven from the FIG. 1 position to to the right as viewed in FIG. 1, pin54 will be disengaged from abutment shoulder 72 as soon as this movementstarts. However, carrier 68 will remain in the FIG. 1 position andproximity switch 10 will remain in the switch off condition. As pistonrod 32 approaches its fully retracted position, pin 54 will move intoengagement with abutment shoulder 70 of carrier 68 and shift the carrierto the right as viewed in FIG. 1 to move target 74 toward coil 120. Thespacing between abutment shoulders 70 and 72 is selected such that pin54 will not engage shoulder 70 until piston rod 32 is within arelatively short distance of its fully retracted position. During thisfinal stage of movement to fully retracted end limit, pin 54 will engageabutment shoulder 70 and drive carrier 68 to the right a sufficientdistance to move target 74 into actuating range of coil 120. As soon astarget 74 moves into range of coil 120, the proximity switch 10 shiftsto its switch on position. Upon subsequent return of piston rod 32 toits fully extended end limit of movement, a similar action occurs, thatis, switch 10 remains in its on position until, near the end of itsstroke, pin 54 engages abutment shoulder 72 and returns carrier 68 toits switch off position as piston rod 32 arrives at its fully extendedposition.

Referring to FIG. 6, circuit diagram 92 of proximity switch 10 isillustrated. The applicant has found that the apparatus describedhereinabove permits extremely accurate positioning of target 74 withrespect to coil 120, permitting a minimum sensing distance The use of anair coil 120, lacking a ferrite core which can saturate, provides almostunlimited magnetic field immunity. Because coil 120 is substantiallyenclosed in metallic housing 58, it is protected from external sidetargets.

Various aspects of overall operation of proximity switch 10 is thesubject of a separate copending application filed concurrently herewithin the names of John Groves, Joseph Lazzaro, Gregory Nadolski and DonaldVan Zeeland, U.S. Ser. No. 133,755, 12-16-87, the specification of whichis incorporated herein by reference.

FIG. 8 shows the percent reduction of the equivalent tank impedance atresonance, (Rp) as the steel target 74 approaches the front surface ofcoil 120. This percent reduction is commonly called the transducer gain.In the application, target 74 moves from almost flush with the 1 mmbarrier 128 protecting coil 120 to 2.5 mm from the barrier. Thistranslates to a movement of 1 mm to 3.5 mm from the front surface of thecoil 120. Curves are plotted for both the shielded and unshieldedversions. The presence of the copper shield degrades the gain somewhatbut provides a great deal of immunity from side targets if nonmetallichousing 58 is used. FIG. 8 represents experimental data taken without ametallic housing with a 0.75×1×0.25 inch copper shield flush with thefront surface of a 300 turn 34 gauge coil operating at 140 KHz.

Referring to FIG. 6, lead 124 of coil 120 is connected to input terminalVI of a proximity switch integrated circuit 130. Lead 126 of coil 120 isconnected to ground. The operation of circuit 130 is described in U.S.Pat. No. 4,543,527, the specification of which is incorporated herein byreference. Input terminal VI of IC 130 is interconnected to groundthrough a 2.2 nanofarad capacitor 132. Terminals XIV and VII of IC 130are connected directly to ground and terminal IV is interconnected toground through a 0.01 microfarad detector capacitor 134 which maintainstank oscillation amplitude. Terminal V of IC 130 is interconnected toground through a series combination of a 8.2k resistor 136, the fixedresistance portion of a 2k potentiometer 138 and a 330 ohm resistor 140.Terminal XI of IC 130 is interconnected to the point of commonconnection of potentiometer 138 in resistor 140 as well as the wiper ofpotentiometer 138. The inputs from connector 90 are electricallyconnected to the AC input terminal of a 400 volt type VM48 rectifiermanufactured by Varo 142. The AC terminals of rectifier 142 are bridgedby a 220 volt varistor 144 for transient protection. The negative outputterminal of rectifier 142 is connected to ground and to the remainingterminal of connector 90. The positive output terminal of rectifier 142is interconnected to the emitter of an MPSA92 transistor 146 through aparallel combination of a 5.1 ohm resistor 148 and a 10 ohm resistor150. Transistor 146 is connected as part of a Darlington pair with asecond transistor 152 with the common collectors thereof connected toground through a reverse biased 7.5 volt 1 watt zener diode 154. Thepositive output terminal of rectifier 142 is also directly connected tothe emitter junction of a type MPSA63 transistor 156 and to the base oftransistor 156 through a parallel combination of an 18k ohm resistor 158and a 0.001 microfarad capacitor 160. The base of transistor 156 isinterconnected with the base of transistor 152 and the collector oftransistor 156 through a series combination of a 10k ohm resistor 162and a forward biased diode 164. The base of transistors 156 and 152 arecommonly connected to the collectors of the Darlington type MPSA42transistor pair 166 and 168. The emitter junction of transistor 168 isinterconnected to ground through a series combination of a 1.5k ohmresistor 170 and a green type HLMP1790 LED 172.

The point of common connection between resistor 162 and the anode ofdiode 164 is interconnected to the emitter junction of a type 2N3906transistor 174 through a 56k ohm resistor 176. The emitter of transistor174 is interconnected to ground through a series connected type HLMP1700red LED 178 and a 2.2 microfarad electrolytic capacitor 180. Note thatLEDs 100 and 102 of FIG. 5 correspond with LEDs 172 and 178 of FIG. 6,respectively. The cathode of LED 178 is connected to the cathode ofzener diode 154, to the collector of transistor 174, to terminal III ofIC 130 and interconnected to terminals I and XIII of IC 130 through a330k ohm resistor 182. The base of transistor 166 is electricallyconnected to output terminal II of IC 130 and interconnected to the baseof transistor 174 through a 100k ohm resistor 184.

Potentiometer is employed to set nominal trip distance at 2.2 mm. Theoutput of IC 130 pin II is held low when the oscillator s active i.e.target 74 remote from coil 120. As target 74 is moved towards coil 120,the oscillator is damped and the output at pin II is released andallowed to go high. When this happens, Darlington transistors 166 and168 switch on green LED 170, pulling base current through the Darlington146, 152 output stage, thus causing load current to flow through bridge142 the output stage and zener 154. Transistor 156 is employed tomonitor the voltage drop across the 3.3 ohm sampling resistor (148 and150) and the two output stage V_(be) drops. If the load current becomesexcessive the cathode of diode 164 drops sufficiently to conduct andthus de-energize the output stage regeneratively. Current flows throughthe overload LED 178 since the LED bypass transistor 174 is de-energizedbecause pin II is still high. When the overload is removed, the outputstage will again operate normally. With the value shown, the output israted for 100 ma before overload takes place.

Results of the variation of trip distance versus temperature areillustrated in FIG. 9. The trip distance was initially set at 4 mm (roomtemperature) which represents a transducer gain of 15%. The response ofFIG. 9 is based upon a 300 turn coil of 34 gauge wire. Based upon theslope of the curve, a smaller wire size (35 gauge) will rotate thetemperature response counterclockwise as illustrated in FIG. 10. Thetemperature response for the coil of FIG. 10 is taken for a far sensedistance and in FIG. 11 for a moderate sense distance. Low temperatureoperation has been improved using 35 gauge wire but high temperatureperformance has been degraded. Note that the curve of FIG. 11 flattensout as the unit is calibrated for closer trip distances. This isbelieved to be because of the higher gain of the sensed coil 120. At theanticipated trip distance of 2.2 mm in the actual application, the curvemay be flatter yet.

In the cylinder application, higher temperature performance is probablypreferred. In that case, 34 gauge wire would be a proper choice. If goodperformance is required over the entire temperature range, then 35 gaugewire would be chosen for its low temperature performance and athermistor network would be added in series with the distance resistor,pin V of IC 130, to improve the high temperature performance.

It is to be understood that the invention has been described withreference to a specific embodiment to provide the features andadvantages previously described and that such specific embodiments aresusceptible of modification, such as will be apparent for those skilledin the art. Accordingly, the foregoing is not to be construed in alimiting sense.

I claim:
 1. A proximity switch for use with a power actuated deviceincluding a member mounted for movement between predetermined limits oftravel, said switch comprising:means operative to change conductivestate in response to a control signal; a substantially closed housingassembly defining a cavity therein, said housing assembly includingmeans operative to shield at least a portion of said cavity fromexternal electromagnetic fields; retainer means formed of non-ferrous,electrical insulating material nestingly received within said cavity anddefining a coil receiving recess within said shielded cavity portion; agenerally toroidal, open core inductive circuit element disposed in afixed, predetermined orientation within said recess; means operative toelectrically energize said inductive element to establish a flux fieldhaving a characteristic amplitude thereabout substantially within saidshielded cavity portion; target means disposed substantially within saidshielded cavity portion and displaceable along an axis of said inductiveelement between a position remote from said inductive element whereinsaid target means is substantially outside of said flux field and asensed position adjacent said element wherein said target means iswithin said flux field to effect a change in said characteristicamplitude; circuit means operative to generate said control signal as afunction of said change in characteristic amplitude; and meansoperatively engaging said target means to effect displacement thereof inresponse to said member movement.
 2. The switch of claim 1, wherein saidretainer means is constructed of dielectric plastic material.
 3. Theswitch of claim 1, wherein said circuit means includes a plurality ofdiscrete circuit elements retained in-circuit upon a generally planarcircuit board disposed within said housing cavity.
 4. The switch ofclaim 3, wherein said retainer means is operative to retain said circuitboard in a predetermined fixed orientation within said housing cavity.5. The switch of claim 4, wherein at least one of said discrete circuitelements is a light processing element, wherein said housing assemblyincludes an optically transmissive window and wherein said retainermeans is operative to position said light processing element in spacedalignment relation with said window.
 6. The switch of claim 5, whereinsaid light processing element comprises a light emitting diode.
 7. Theswitch of claim 4, wherein said retainer means is operative toelectrically insulate said circuit board and discrete circuit elementsfrom said housing assembly.
 8. The switch of claim 2, wherein saidretainer means comprises a barrier portion between said inductivecircuit element and target means
 9. The switch of claim 8, wherein saidbarrier portion is laterally substantially coextensive with the lateraldimension of both said inductive circuit element and target means. 10.The switch of claim 8, wherein said barrier portion has a relativelythin, substantially uniform thickness as measured in the line ofdisplacement of said target means
 11. The switch of claim 10, whereinthe dimensional displacement (D_(T)) of said target means between saidremote and sensed positions substantially exceeds said barrier thickness(T_(B)), i.e. (D_(T) >T_(B)).
 12. The switch of claim 11, wherein D_(T)≧2.5T_(B).
 13. The switch of claim 11, wherein the dimensionaldisplacement of said power actuated device (D_(D)) substantially exceedsD_(T), i.e. D_(D) >D_(T).
 14. The switch of claim 11, wherein said meansoperatively engaging said target means effects a lost motion betweensaid target means and power actuated device whereby said target meanstransitions between said remote and sensed position only as said memberalternatively approaches said predetermined limits of travel.
 15. Theswitch of claim 3, wherein said retainer means comprises a plurality ofbody portions and at least one retention tab depending from one of saidbody portions, said retention tab including a ramp portion operative tomomentarily displace said retention tab laterally outwardly uponengagement with an edge of said circuit board during assembly and alaterally extending step portion operative to entrap said edge betweensaid retention tab and associated body portion.
 16. The switch of claim1, wherein said inductive element comprises an air coil wherein saidflux field is disposed substantially within the atmosphere surroundingsaid coil, said atmosphere comprising a substantially air filled voidwithout ferrous material components.
 17. The switch of claim 1, whereinsaid target means comprises a generally planar ferrous metal memberdisplaceable along said axis.
 18. In combination:a power actuated deviceincluding a rigid member mounted within a fixed frame for movement alonga fixed path relative to said frame between a first end limit to traveland a second end limit to travel, power actuated means for driving saidrigid member in either direction from one of said end limits to theother; and a proximity switch for signaling the arrival of said memberat either of said end limits, said switch comprising, means operative tochange conductive state in response to a control signal, a substantiallyclosed housing assembly fixedly mounted upon said frame in surroundingrelationship to a slot extending through said frame along said fixedpath said housing assembly defining a cavity therein and including meansoperative to shield at least a portion of said cavity from externalelectromagnetic fields, retainer means formed of non-ferrous,electrically insulating material nestingly received within said cavityand defining a coil receiving recess within said shielded cavityportion, a generally annular, open core inductive circuit elementfixedly mounted in a predetermined orientation within said recess, meansoperative to electrically energize said inductive element to establish aflux field having a characteristic amplitude thereabout substantiallywithin said shielded cavity portion of said housing, a proximity switchtarget member mounted within said shielded cavity portion for slidingmovement parallel to said path along an axis of said circuit elementbetween a first position wherein said target member is within actuatingproximity of said inductive element and a second position wherein saidtarget member is out of actuating proximity to said inductive element byeffecting changes in said characteristic amplitude, circuit meansoperative to generate said control signal as a function of said changein characteristic amplitude, first abutment means on said rigid member,and second abutment means on said target member, one of said abutmentmeans comprising means on one of said members defining an elongaterecess terminating at opposite ends in abutment shoulders normal to saidfixed path and the other of said abutment means comprising a projectionon the other of said members projecting from said other of said membersnormal to said fixed path through said slot in said frame into therecess of said one of said members between said abutment shoulders, saidshoulders being spaced from each other by a distance such that uponmovement of said rigid member to said first end limit one of saidshoulders engages said projection and pushes said target member intosaid first position and upon movement of said rigid member to saidsecond end limit the other of said shoulders engages said projection andpushes said target member into said second position.
 19. A proximityswitch for use with a power actuated device including a member mountedfor movement between predetermined limits of travel, said switchcomprising:means operative to change conductive state in response to acontrol signal; a substantially closed housing assembly defining acavity therein, said housing assembly including means operative toshield at least a portion of said cavity from external electromagneticfields; retainer means nestingly received within said cavity anddefining a coil receiving recess within said shielded cavity portion; aninductive circuit element disposed within said recess; means operativeto electrically energize said inductive element to establish a fluxfield having a characteristic amplitude thereabout substantially withinsaid shielded cavity portion; target means disposed substantially withinsaid shielded cavity portion and displaceable between a position remotefrom said inductive element and a sensed position adjacent said elementto effect a change in said characteristic amplitude; circuit meansoperative to generate said control signal as a function of said changein characteristic amplitude; and means operatively engaging said targetmeans to effect displacement thereof in response to said membermovement, said circuit means including a plurality of discrete circuitelements retained in-circuit upon a generally planar circuit boarddisposed within said housing cavity, said retainer means comprising aplurality of body portions and at least one retention tab depending fromone of said body portions, said retention tab including a ramp portionoperative to momentarily displace said retention tab laterally outwardlyupon engagement with an edge of said circuit board during assembly and alaterally extending step portion operative to entrap said edge betweensaid retention tab and associated body portion, and said housingassembly including laterally spaced wall portions disposed, upon finalassembly of said proximity switch, in close lateral proximity to saidretention tab and a body portion disposed laterally opposite therefromwhereby said retention tab is thereafter restrained from further outwardlateral displacement.
 20. A proximity switch for use with a poweractuated device including a member mounted for movement betweenpredetermined limits of travel, said switch comprising:means operativeto change conductive state in response to a control signal; asubstantially closed housing assembly defining a cavity therein, saidhousing assembly including means operative to shield at least a portionof said cavity from external electromagnetic fields; retainer meansnestingly received within said cavity and defining a coil receivingrecess within said shielded cavity portion; an inductive circuit elementdisposed within said recess; means operative to electrically energizesaid inductive element to establish a flux field having a characteristicamplitude thereabout substantially within said shielded cavity portion;target means disposed substantially within said shielded cavity portionand displaceable between a position remote from said inductive elementand a sensed position adjacent said element to effect a change in saidcharacteristic amplitude; circuit means operative to generate saidcontrol signal as a function of said change in characteristic amplitude;and means operatively engaging said target means to effect displacementthereof in response to said member movement, said circuit meansincluding a plurality of discrete circuit elements retained in-circuitupon a generally planar circuit board disposed within said housingcavity, and said retainer means including a plurality of body portionsand at least one retention tab depending from one of said body portions,said retention tab including a ramp portion operative to momentarilydisplace said retention tab laterally outwardly upon engagement with anedge of said circuit board during assembly and a laterally extendingstep portion operative to entrap said edge between said retention taband associated body portion.